Efforts to economically develop offshore oil and gas fields in ever deeper water create many unique engineering challenges. One of these challenges is providing a suitable surface accessible structure. Spars provide a promising answer for meeting these challenges. Spar designs provide a heave resistant, floating structure characterized by an elongated, vertically disposed hull. Most often this hull is cylindrical, buoyant at the top and with ballast at the base. The hull may be anchored to the ocean floor through risers, tethers, and/or mooring lines.
Though resistant to heave, spars are not immune from the rigors of the offshore environment. The typical single column profile of the hull is particularly susceptible to VIM problems in the presence of a passing current. These currents cause vortexes to shed from the sides of the hull, inducing vibrations that can hinder normal drilling and/or production operations and lead to the failure of the anchoring members or other critical structural elements.
Helical strakes and shrouds have been used or proposed for such applications to reduce vortex induced vibrations. Strakes and shrouds can theoretically be made to be effective regardless of the orientation of the current to the marine element, however, practice has shown that ineffective “bald spots” may occur in the vicinity of the strake terminations. But shrouds and strakes materially increase the drag on such large marine elements.
U.S. Pat. No. 6,227,137 discloses a spar platform having a deck supported by a buoyant tank assembly having a first buoyant section connected to the deck, a second buoyant section disposed beneath the first buoyant section; and a buoyant section spacing structure connecting the first and second buoyant sections in manner providing a horizontally extending vertical gap therebetween. A counterweight is connected to the buoyant tank assembly through a counterweight spacing structure. U.S. Pat. No. 6,227,137 is herein incorporated by reference in its entirety.
There is a need in the art for a low drag, VIM reducing system suitable for deployment in protecting the hull of a spar type offshore structure.